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creases to a maximum goes far beyond the technological problem
that gave rise to thermodynamics. Increasing entropy is
no longer synonymous with loss but now refers to the natural
processes within the system. These are the processes that ultimately
lead the system to thermodynamic "equilibrium"
corresponding to the state of maximum entropy.
In Chapter I we emphasized the element of surprise involved
in the discovery of Newton's universal laws of dynamics.
Here also the element of surprise is apparent. When Sadi
Carnot formulated the laws of ideal thermal engines, he was
far from imagining that his work would lead to a conceptual
revolution in physics.
Reversible transformations belong to classical science in the
sense that they define the possibility of acting on a system, of
controlling it. The dynamic object could be controlled through
its initial conditions. Similarly, when defined in terms of its
reversible transformations, the thermodynamic object may be
controlled through its boundary conditions: any system in
thermodynamic equilibrium whose temperature, volume, or
pressure are gradually changed passes through a series of
equilibrium states, and any reversal of the manipulation leads
to a return to its initial state. The reversible nature of such
change and controlling the object through its boundary conditions
are interdependent processes. In this context irreversibility
is "negative"; it appears in the form of "uncontrolled"
changes that occur as soon as the system eludes control. But
inversely, irreversible processes may be considered as the last
remnants of the spontaneous and intrinsic activity displayed
by nature when experimental devices are employed to harness
it.
Thus the "negative" property of dissipation shows that, unlike
dynamic objects, thermodynamic objects can only be partially
controlled. Occasionally they "break loose" into
spontaneous change.
All changes are not equivalent for a thermodynamic system.
This is the meaning of the expression dS =deS+ diS.
Spontaneous change toward equilibrium diS is different from
the change deS, which is determined and controlled by a modification
of the boundary conditions (for example, ambient
temperature). For an isolated system, equilibrium appears as